1. Field of Invention
This invention is related to piezoelectric actuated suspensions with passive damping, specifically to such suspensions are used for positioning precision and shock resistance in dual-stage hard disk servo systems.
2. Prior Art
Positioning precision and shock resistance are crucial to today's increasingly high-speed, high-capacity, high data density, and miniaturized hard disk drives. The main operation of a hard disk drive is to store and retrieve the bit data by positioning a magnetoresistive read/write head at a specific target area over a rotating magnetic data storage platter at high speed. To store the information, the read/write head writes the bit data onto the platter by aligning magnetic poles set in concentric data tracks on the platter. And so as to read the information stored, the head will sense the change of the magnetic flux on the platter. It excites the current in the head coil and the read voltage pulses at the flux transitions are then translated into sequences of data bits. In order to increase the recording density and data storage of disks, the bit length on the track and track width are shrinking further. Besides shrinking the bit length on the track, decreasing in the track width becomes more important to meet the requirement of high capacity and high data density hard disk drives. The higher the track density on a platter, the more data can be stored on a given hard disk drive.
In a typical disk drive, the magnetic heads are supported and held above the disk surfaces by a voice coil motor (VCM) arm. As the VCM moves back and forth, the sliders are positioned over the surfaces of the disk and the read/write heads on the sliders are held at an exact position relative to the surfaces for reading or writing; furthermore, they are able to move from one track to another track to allow access to the entire surfaces of the disk. However, since the operational speed becomes faster and the track width becomes thinner, the conventional single stage VCM is difficult to meet the demands due to its mechanical limitations. The problem of track misregistration due to track runout would lead to a track seeking or following error during read/write operation.
Therefore, the demand for higher bandwidth servo systems that can quickly and precisely position the read/write head on a high track density becomes more pressing. In recent years, the idea of applying dual-stage actuators to track servo systems has been developed to solve the limitations of traditional servo system design such as U.S. Pat. No. 6,760,194 B2 (2004) and U.S. Pat. No. 6,680,826 B2 (2004) to Shiraishi et al., and U.S. Pat. No. 6,404,600 B1 (2002), U.S. Pat. No. 6,421,211 B1 (2002), U.S. Pat. No. 6,512,659 B1 (2003), U.S. Pat. No. 6,807,030 (2004) to Hawwa et al.
In particular, push-pull piezoelectric actuated devices have been developed as a fine actuator for the servo system, while the voice coil motor functions as a coarse actuator. As shown in FIG. 1, in a current disk drive 100, the first stage VCM arm 110 is controlled and turned around the pivot bearing 108. It positions the read/write head 102 to rest on the target tracks 116 for read/write operation. The data information is stored or read from the disk platters 112, which are fixed on the spindle 114 and rotate at high speed. The bit data and control signals are transmitted to and from the electronics block through the flex circuit 118. The second stage actuator will replace the original suspension 104, which is connected to the E-block 106.
In the prior art, the original suspension 104 can be replaced by a push-pull piezoelectric actuated device 20. The device is mounted on a specially fabricated suspension 28 by an adhesive. In the device, one piezoelectric actuator 22A contracts and pulls the suspension 28 in the longitudinal direction while the other piezoelectric actuator 22B expands and pushes the suspension 28 in the longitudinal direction. As the base plate 26 is fixed on the E-block 106, the load beam swings back and forth around the hinge 24. The motion induced from the actuators is amplified by a lever mechanism about the pivot at hinge 24. Then this in-plane movement is imparted to the slider flying on the disk surface. It cooperates with the movement of the VCM to form a dual-stage servo system.
In hard disk drives, runout refers to the deviation of the read/write head from the target track, which is one of the main contributors to track misregistration. There are two kinds of runout: repeatable runout (RRO) and non-repeatable runout (NRRO). The RRO is a periodic signal and is phase locked to the spindle rotation. NRRO has characteristic frequencies, but it is not locked to a particular location on the disk. For high density data recording and high-speed disk rotation, the NRRO factors that affect the precision positioning of the read/write head are the vibrations of disk and spindle assembly, head carriage arm, and suspension subjected to the airflow disturbance. The excitation force comes from the spatiotemporal fluctuation of pressure generated mainly by the disturbance of airflow onto the disk and arm surface. On the other hand, better shock resistance is demanded for the hard disk drives under sudden excitations such as during shipping and read/write operations.
However, the shock resistance of the prior dual-stage piezoelectric actuators may not be good enough because they are using piezoelectric patches only such like the manner in the prior art of FIG. 1.